1. Field of the Invention
The invention relates to an ignition device for an internal combustion engine.
2. Description of the Related Art
In an internal combustion engine, it is necessary to unfailingly ignite homogeneous air-fuel mixture in the entire space in a cylinder or air-fuel mixture in a part of the cylinder using a spark plug. However, a conventional spark plug, which generates a spark in a spark gap, ignites the air-fuel mixture only at one point, and thus, the conventional spark plug has relatively low ignitability.
An ignition device that employs the plasma jet injection has been proposed for improving the ignitability (for example, refer to Japanese Patent Application Publication No. 2006-294257 (JP-A-2006-294257)). The ignition device includes a chamber, an outer electrode, and a center electrode. The chamber is provided with an opening portion to be open to the cylinder and a bottom surface that is disposed facing the opening portion, the chamber extending in the axial direction. The outer electrode is disposed around the opening portion, and the center electrode is disposed to provide the bottom surface in the chamber. The plasma is produced in the chamber by applying voltage between the center electrode and the outer electrode. In the thus-configured ignition device, the plasma jet is injected through the opening portion of the chamber so that the part of the air-fuel mixture over a predetermined area that corresponds to a sectional area of the injected plasma jet is simultaneously ignited. Therefore, the ignition device improves the ignitability.
The ignition device described in Japanese Patent Application Publication No. 2006-294257 (JP-A-2006-294257) includes a metal housing that is formed integrally with or separately from the outer electrode. The center electrode is supported by an insulating member, insulated from the housing and the outer electrode, and the insulating member forms a peripheral wall of the chamber. A screw thread is formed on a periphery of the housing so that the ignition device is screwed into a cylinder head, and a gasket is disposed in an outer side than the screw thread. Therefore, a relatively long portion of the ignition device, including the screw thread, that is positioned in a side closer to the inside of the cylinder than the gasket is exposed to the high-temperature combusted gas in the cylinder, which results in an insufficient heat dissipation from the insulating member. Therefore, in order to ensure the heat resistance of the ignition device, the thickness of the insulating member that forms the peripheral wall of the chamber needs to be relatively thick.